Update hc.front.views.cron_preview to catch CronSimError explicitly
Update hc.front.views.cron_preview to catch CronSimError explicitly

With a catch-all "except:" rule, we would swallow any unexpected
exceptions (ValueError, etc.) in cronsim. But we want to know
about them. cron_preview is a place where we can afford to crash, and generate a crash report.

iphoting/healthchecks,healthchecks/healthchecks,iphoting/healthchecks,iphoting/healthchecks,healthchecks/healthchecks,iphoting/healthchecks,healthchecks/healthchecks,healthchecks/healthchecks